You can access the area he is found in via the hook-on-a-chain or whatever its called in english. He tells you something like " Ah, nice to see u pay me a visit" and then he attacks, as I transform he says something like "oh you brought a friend" and dies shortly after under my blows. No further explanation, no reward of any kind, no new passage opening or anything like that.

And there the apocalyptic horseman of war stands in some itzy bitzy cave and mumbles: "What the hell? Who was that guy and what was he all about anyway?"